The invention relates to a process for manufacturing open-air compound insulators, particularly for use in areas having a high degree of atmospheric pollution.
Compound insulators have been known for a long time. Usually they comprise a mechanically high-strength, fiber reinforced synthetic core for absorbing mechanical loads, on which screens for avoiding weather factor produced electric spark-overs and suspension fittings for attaching insulators to transmission towers are mounted.
Compound designs of high-voltage open-air insulators have a number of considerable advantages over conventional insulators made of porcelain and glass which can be traced back to their design. With compound type insulators, electrical and mechanical functional area matching materials are used so that insulators as these can be economically feasible with a minimal material input. For this reason, compound insulators can be manufactured weighing substantially less than conventional insulators. They are more impact resistant than the latter, and maximal force applications are feasible. For use under maximal stresses, compound insulators can be designed also in one piece, which because of the low weight involved facilitates the design of open-air transmission towers.
But even high-voltage open-air compound insulators have their share of constructional and material selection problems. Because the compound zone between rod and screens is exposed to considerable electrical loads--the reason for this being that is runs parallel to the electrical field--measures must be taken for the prevention of electric sparkovers. Additionally, the coefficients of expansion of fiber reinforced rod and screens must be considered because in extreme cases, the operating temperatures may fluctuate between -50.degree. C. and +80.degree. C. Also wind and ice produced tractive forces, oscillations and abruptly added loads and their unloading effects produce additional tensions in the compound zone, which can result in electric sparkovers. Furthermore, weather and pollution produced moisture can penetrate the insulator. Specifically, electrical partial discharges on insulator surfaces require a specifically selected screen material, so that in avoiding operational sparkovers, no creep stresses are formed. Furthermore, a maximal type of operational safety and reliability ranging over decades is expected from high-voltage insulators so that the material selection, design and manufacturing of compound insulators must be very carefully done taking into account the economic aspects.
To solve this complex problem a large number of materials, designs and manufacturing processes have been practiced. Thus patent 963,115 proposes a compound insulator where the carrier rod is provided with a coating of fluorocarbon resin, and a screen made of a conductive material is attached. This insulator is not suitable as an open-air insulator for although the insulator stalk is partially protected against rain, or any open-air pollution of the insulator the stalk surface becomes partially conductive so that because of the omission of insulating material screens, which limit the effect of partial arc-overs, sparkovers can occur on the insulator.
British Pat. No. 1,292,276 describes an improved compound insulator made of a centrally arranged carrier, the outer surface of which is enveloped by a creep resistant material, over which thermally shrinkable prefabricated screens are slipped, which consist of a creep resistant material, and attached to the envelope of the carrier by means of heat meltable compounds. A substantial drawback of this proposed thermal shrink-on process is that the contraction strain of partially thermoplastic-formable materials is so minor that no pressing power can be effective between slipped-over screen and carrier envelope; as a result, small hollow spaces and gaps remain in the joint packing material so that diffused-in water can condense and result in electric spark-overs. This applies also to the coating on the carrier, which is attached in the same manner and means as the screens. Furthermore, the proposed material is very expensive and requires a high processing input.
Another process is described in DOS No. 2,254,468 U.S. Pat. No. 3,735,019, in which mutually overlapping butyl rubber screens are attached along the axis of the central, elongated main tube. The screens are prefabricated and slipped over said main tube by means of silicone grease. The drawback of this insulator is that the screens overlap each other, i.e., any leeway given in shaping the insulator is limited. Using this way to meet a requirement for more screens on the same insulator length, specifically on using said insulator under severe-polluted atmospheric conditions, requires an expensive second mold for manufacturing other screens. Even on optimizing the insulator for use in areas less endangered by external layers of pollution, where only a few screens are needed, again a new screen mold is required. Furthermore, under open-air conditions, the proposed butyl rubber screen material is susceptible to autoxidation because of any present double bonds, which reduces creep resistance. Also the proposed silicone grease intermediate layer is not resistant to saponfification; in an electrical field, the silicone grease is decomposed by water diffused into the butyl rubber, so that the conductive products can initiate an electric sparkover between screen and carrier.
A further proposal published in British Pat. No. 915,052 is targeted to provide a glass fiber rod with a layer of creep resistant material, e.g., neoprene, butyl or silicone rubber, and with fluorocarbon resins. Proposed there is also that the layer be applied in an extrusion process. Further proposed as an alternative was also that elastically expanded tubes made of this material having an inner diameter less than that of the glass fiber rod be slipped over the glass fiber rod. So that no moisture can penetrate between rubber coating and suspension fitting, the material joint between rubber coating and suspension fitting is covered by a further flexible tube piece. It has been proposed also that on using silicone rubber on the material joint between the suspension fitting and rubber layer, a coil made of an elastomeric material and of a cotton tape be used as seal, whereby beneath the coil a thin elastomeric coating is present on the roughened, primer-pretreated silicone surface. But the proposed insulator has considerable drawbacks. The proposed measures taken for sealing the material joint between suspension fitting and rubber layer on the rod are ineffective. Because of the electrical field produced between the suspension fitting, an increased water vapor diffusion through the coating on the rod takes place. Because of the omission of an electrically leakproof transition layer, microsize hollow spaces are present in which water can condense, so that unavoidably joint sparkovers occur. This is not prevented even by the measure taken of shaping the rubber layer on the rod by power pressing or extruding it on because in any case, minimal gaps and hollow spaces cannot be avoided. The main drawback of the proposed insulator is a complete lack of screens so that the available standard insulator design-length rated creep age path is not sufficient at all. Moreover, an extension in length by a multiple of standard screen insulator length is required so that compared with screen insulators, a highly uneconomical design results.
The object of the invention, therefore, is to provide a manufacturing process of sparkover-proof compound insulators, whereby according to above outlined parameters, the above indicated design problems will also be eliminated from this type of insulator. An economic process of manufacturing such insulators is found by a selection of partially known processing steps and materials. Furthermore, sparkover-proof compound insulators are suitable for use specifically in highly polluted areas.
This problem is solved according to the invention by subjecting a prefabricated glass fiber rod to a surface treatment with silanes, by applying a rubber layer to the prepared glass fiber stalk by extrusion means, by strengthening the rubber layer, by slipping previously radial-expanded, prefabricated screens over the extruded coating, by vulcanizing, and by the subsequent attaching of fittings to the end of the glass fiber reinforced stalk.
Further advantages, features and potential applications of the novel invention are evident from exemplified embodiments and the following description.